The disclosed invention generally relates to the joining of plastic piping system component connection ends, and is more particularly directed to apparatus for fusion joining plastic piping system component connection ends for use in high purity applications where internal voids, crevices, cavities, or the like cannot be tolerated.
Plastic piping system components made from a variety of thermo-plastic materials are utilized in a wide range of applications as a result of the useful characteristics of thermo-plastic materials, which include resistance to corrosion, imperviousness to contamination, reduced cost, ease of installation, and lighter weight.
Known methods of joining plastic piping system components include the use of mechanical means, solvent cement, and/or heat fusion.
Examples of mechanical means include threaded couplings, compression fittings, and flanged fittings. An example of solvent cement joining would be the application of cement to a pipe end which is then inserted into a fitting or valve. An example of heat fusion is a butt joint between piping system component ends. Such butt joints tend to be weak, and various couplings have been devised for joining piping system components through the application of heat. A further example of heat fusion is socket fusion where heated piping system component ends are inserted into a coupling.
In high purity applications, such as the semiconductor industry, the food industry, the pharmaceutical industry, the biotechnical industry, and chemical manufacture, it is important that contamination be avoided. Mechanical joining techniques have been utilized in high purity applications, but require frequent disassembly for cleaning since the mechanical joining components have crevices and voids where contamination can be readily lodged.
The use of solvent cement presents a contamination problem since internal voids, crevices, or cavities are readily formed. Contamination or potentially contaminating material could readily be lodged in such voids, crevices, or cavities.
The use of known heat fusion joining techniques in high purity applications also present a contamination problem in that unwanted voids, crevices, or cavities are readily formed. Contamination or potentially contaminating material could become lodged in such voids, crevices, or cavities. As a result, attempts have been made to provide for heat fusion joints which are smooth and free of voids, crevices and cavities on the inside. Such attempts have included the use of some form of inside support apparatus to support the piping system component connection ends when they are subjected to heat in the fusion process. For example, expandable mandrels which are inserted and then expanded to the inside diameter have been utilized. An important consideration with the use of internal support apparatus is the planning required to make sure that the support apparatus could be removed after the piping system components are joined. For example, if the piping system components being joined include right angle joints that have already been formed, removal could pose a problem. Also, the fact that support apparatus is inside the piping system components being joined poses the potential for contamination.
Another consideration with known heat fusion techniques is that they cannot be used with readily oxidized thermo-plastic materials such as polyvinyl chloride (PVC).